Over half of the acute respiratory tract illness in infants and young children can presently be related to respiratory syncytial virus, a parainfluenza virus, an adenovirus, an influenza virus or Mycoplasma pneumoniae. The role of other agents apparently is much less important. The epidemiology of the more common agents is becoming increasingly understood, but often has shown surprising variations. Little is known regarding the pathogenesis of infection with these agents, particularly with respect to the factors of host resistance, and the role of local, specific cellular, or serum antibody in protection or in production of disease. In recent studies human rotavirus infection has been detected in approximately 50% of young infants and children with acute diarrhea requiring hospitalization. A majority of these infections occurred between November and April. It is important to learn whether this etiologic link and epidemiologic pattern will continue and what the relative role of toxigenic bacteria and adenovirus may be. In order fully to understand these infections and their possible prevention we propose to continue to study: the relationship of non-bacterial agents to pediatric respiratory tract and gastrointestinal illness and the relationship of toxigenic bacteria to the latter; the pathogenesis of infection in development of resistance to respiratory tract and gastrointestinal infection; and the natural history of the infecting agents. A new emphasis will be placed on more sensitive and/or more rapid methods for detection of these agents and serologic responses to them.